Microelectronic devices (semiconductor devices) are fabricated by repeating the photolithography technology of irradiating exposure light to a pattern-transfer mask, typically a photomask having a circuit pattern drawn thereon, for transferring the circuit pattern to a semiconductor substrate or wafer through a reduction projection optical system. As the circuit pattern of semiconductor devices is successively miniaturized, the mainstream exposure light now becomes the argon fluoride (ArF) excimer laser of wavelength 193 nm. Now that the multi-patterning process of combining exposure steps and processing steps plural times is adopted, a pattern having a feature size which is fully smaller than the exposure wavelength can be eventually formed.
The pattern-transfer mask is manufactured by forming a circuit pattern on an optical film-bearing substrate (mask blank). The optical film (thin film) is generally a film based on a transition metal compound or a film based on a transition metal-containing silicon compound. Depending on a particular purpose, a film functioning as light-shielding film or a film functioning as phase shift film is selected. Also included is a hard mask film which is a processing aid film intended for the high-precision processing of an optical film.
Since pattern-transfer masks, typically photomasks is used as the original for the fabrication of semiconductor devices having a fine size pattern, the masks must be defect-free. This naturally requires that photomask blanks be defect-free. Circuit patterns are formed by depositing a resist film on a film-bearing photomask blank, processing the resist film and underlying film by the conventional lithography or EB lithography to finally form the pattern. In this sense, the resist film is also required to have no defects such as pinholes. Under the circumstances, efforts have been devoted to the technology for detecting defects on photomasks or photomask blanks.
Patent Documents 1 and 2 describe methods for detecting defects or foreign particles by irradiating laser light to a substrate and capturing the scattered reflection light. Also described is the technology of providing detection signals with asymmetry and identifying whether defects are bump or pit. Patent Document 3 discloses the inspection of a pattern on an optical mask using deep ultraviolet (DUV) light. Further Patent Document 4 describes the technique of dividing an inspection beam into a plurality of spots, scanning the spots on the substrate, and receiving the reflected beam by a photo-detector. Patent Document 5 discloses the technology of using extreme ultraviolet (EUV) light near wavelength of 13.5 nm for identifying the bump/pit shape of defects on an EUV mask blank.